1. Field of the Invention
The present invention relates to a width measuring device which measures a width parameter, such as a track width of a magnetic head on a disk, at high speed and with high precision by means of an image input device.
2. Description of the Prior Art
Conventional high precision, non-contact width measuring devices which have been practically employed include devices which measure a width by means of image data input by way of a television camera.
One such conventional image measuring device which uses image data is described hereinbelow with reference to the accompanying FIGS. 4 and 5, which are a block diagram of a conventional image measuring device and a flow chart of the operation thereof, respectively.
As shown in FIG. 4, an object 22 to be measured and having parallel characteristic members (CM) is placed on a measurement stand 21. A light 23 for illuminating the object 22 is provided. A television camera 24 is mounted on the end 25 of an actuator of a robot or other device. The television camera 24 is used to input a video image of the object 22 and is positioned perpendicular to the object 22. The TV camera 24 is controlled by a camera control circuit 26.
The video signal input by the TV camera 24 is input to an analog/digital convertor 27 (hereinafter A/D convertor), digitized to graphic data on a scale of 0-255 based on the image density (i.e., converted to a (normally) gray scale image), and this data is input to a microcomputer comprising a CPU, ROM, RAM, and I/O ports.
The image measuring device comprises a detection control circuit (CPU) 29 to which commands are applied from a main controller or from a control panel, a characteristic point (CP) detection circuit 28, a memory control circuit 30, a characteristic member (CM) calculation circuit 31, a parameter memory 32, and a width detection circuit 33. The measurement results are sent to the main controller. An image measuring device so comprised operates as described below.
As shown in the flow chart in FIG. 5, the first step #1 is to place the object 22 to be measured on the stand 21. The object, 22 is then mechanically positioned so that parallel left and right side characteristic members (CM) on the object 22 are perpendicular to the scanning lines of the TV camera 24 (step #1). The TV camera 24 is then focused (step #2), and one scanned image is input (step #3). The characteristic points (CP) at which the multiple scanning lines intersects perpendicularly with the left side characteristic member are detected (step 4) and are stored as X coordinate data. Then, the average of the X coordinates in the resulting characteristic point set is obtained (step 5), and the same operation is then executed for the right side characteristic member (step 6). The distance between the left and right sides resulting from the average calculations performed on the sets of characteristic points is then obtained to determine the width of the object 22 being measured (step 7).
However, with a device as described above, a line representing the edge of one characteristic member of the object being measured is obtained by calculating the average value of a set of characteristic points on an axis perpendicular to said member after calculating the points of the characteristic member of the object 22 intersecting each scanning line set perpendicular to the member. As a result, all characteristic points, including false points far removed from the true points and resulting from data instability factors including contrast and shadows caused by the lighting when the image is obtained by the camera and noise caused by dust and other sources, are used in calculating the characteristic members. This can result in measurements with low reliability.
In addition to the above problem, the processing required to obtain the characteristic points of the characteristic members is time-consuming.